1. Field of the Invention
The present invention generally relates to a multi-level information recording apparatus, a multi-level information recording method and a multi-level information recording medium for using multi-level recording techniques to record information in an information recording medium such as an optical disk and a multi-level information recording-reproducing apparatus for using the multi-level recording techniques to record and reproduce information in an information recording medium such as an optical disk, and more particularly to a multi-level recording apparatus, a multi-level recording method, a multi-level recording medium and a multi-level recording-reproducing apparatus that can determine a multi-level with accuracy even if a recording density is increased.
2. Description of the Related Art
Recent development of computer-related arts and information technologies has accelerated a movement to digitize various types of information such as image information and audio information. In fact, such image information and such audio information are widely distributed as digital information at present. In this circumstance, the larger amount of contents digital information has, the higher recording density an information recording medium for recording the digital information such as an optical disk is required to have. There are various types of optical disks, for instance, CD (Compact Disk), CD-R (Compact Disk Recordable), CR-ROM (Compact Disk Read Only Memory), DVD (Digital Versatile Disk) and so on. In a recordable and readable recording medium such as CD-R and DVD-RAM (Digital Versatile Disk Random Access Memory), information is recorded therein as binary information, that is, 2-level information. In order to increase the recording density along this approach, some conventional methods intend to reduce the pit size of an optical disk or form smaller recorded marks on an optical disk. However, a recording-readable apparatus has limits on wavelength of an illuminant thereof and NA (Numerical Aperture) of an objective lens thereof. Consequently, there are limits on the sizes of pits and recorded marks.
There is another approach for increasing storage capacity of an optical disk without changing the numbers of illuminants and NA of an objective lens. Multi-level information recording methods have been presented in some conferences such as ODS (Optical Data Storage) along this approach. In these multi-level information recording methods, information is recorded as multi-level information, that is, as more than or equal to three-level information rather than binary information.
However, the multi-level information recording methods have some disadvantages in that the level of a multi-level signal for recording multi-level information, which is referred to as a multi-level, is erroneously determined due to inter-symbol interference from adjacent recorded marks.
Japanese Laid-Open Patent Application No. 2001-084592 discloses a multi-level information recording method that intends to reduce multi-level determination errors due to the inter-symbol interference when multi-level information is reproduced. In this multi-level information recording method, when multi-level information or multi-level data is recorded, the average of adjacent recorded marks of a recorded mark is computed. Then, recording power is corrected in proportion to a difference between the average and the recorded mark. According to the multi-level information recording method, the larger difference between the average and the recorded mark is, the more largely the recording power is corrected. In general, the inter-symbol interference is largely caused in proportion to differences between adjacent recorded marks and the recorded mark to be recorded. Therefore, according to the multi-level information recording method, the correction offsets the inter-symbol interference.
However, the above-mentioned multi-level information recording method has some cases where the recording power cannot be successfully corrected.
A description will now be given, with reference to FIG. 1, of the problem on the above-mentioned multi-level information recording method wherein multi-level information is recorded as 8-level information. FIG. 1 shows an observation result of recorded marks according to the 8-level information recording method in a case where a multi-level data sequence “0, 1, 7, 1, 0” is recorded in a phase-change optical disk.
In the conventional 8-level information recording method, it is supposed that each multi-level data “0”, “1”, “7”, “1” and “0” of the multi-level data sequence “0, 1, 7, 1, 0” are recorded in individual cells Se of a track Tr of the phase change optical disk. Regarding a first pattern “0, 1, 7” and a second subsequence “7, 1, 0”, the conventional multi-level information recording method assigns same recording power to the both multi-level data “1”s because the two multi-level data “1”s are in the same condition according to the conventional multi-level information recording method. As is shown in the observation result in FIG. 1, however, the multi-level data “1” and “7” of the first pattern are properly recorded as the recorded marks “m1” and “m7”, respectively, in the cells Se whereas the multi-level data “1” of the second pattern are not successfully recorded as the recorded mark “m” in the cell Se. Through similar experiments on other multi-level data patterns, it is observed that formation of a recorded mark often ends in failure if the recorded mark is located immediately behind a long recorded mark having a higher level than 5-level data in the 8-level information recording.
The above-mentioned problem is mainly caused in the following reason. FIGS. 2A and 2B show waveform diagrams of variations of the recording power under the conventional multi-level information recording method with respect to the first pattern and the second pattern, respectively. When the first pattern is recorded, a laser beam is radiated by varying the recording power or the laser power as radiation energy of the laser beam in a multi-level fashion in accordance with the waveform as shown in FIG. 2A. Similarly, when the second pattern is recorded, a laser beam is radiated by varying the laser power in accordance with the waveform as shown in FIG. 2B.
As is shown in FIG. 3, there are three levels of recording pulses, write power Pw (a top pulse), bias power Pb (a cooling pulse) and erase power Pe (an erase pulse).
When the multi-level data “1” are recorded for the second pattern “7, 1, 0”, a cumulative amount of the laser beam corresponds to the shaded portion shown in FIG. 2B. At this time, the laser beam has the cumulative amount smaller than that of the first pattern with respect to the erase power Pe by the range X shown in FIG. 2B. As a result, the laser beam cannot sufficiently increase the temperature of a recording layer of an information recording medium, and a recorded mark for the multi-level data “1” is not successfully formed.
A detailed description will now be given, with reference to FIGS. 4A and 4B, of 4-level distributions before and after the recording power correction, respectively, according to the above-mentioned conventional multi-level information recording method.
FIG. 4A shows four multi-level distributions before the recording power correction according to the above-mentioned conventional multi-level information recording method. As is shown in FIG. 4A, the multi-level distributions have overlapped areas in the bottom between adjacent multi-level distributions due to inter-symbol interference. When the amplitude of reproduction signals varies in the overlapped areas, the multi-level is erroneously determined, that is, a determination error occurs. If the recording power is corrected in consideration of such amplitude variations of the reproduction signal so that the inter-symbol interference can be offset, deviations of the multi-level distributions becomes smaller as shown in FIG. 4B. As a result, it is possible to suppress the determination errors.
However, if the recording density is further to be increased, the above-mentioned conventional recording power correction method has difficulty of the accurate multi-level determination. The difficulty is explained in detail with reference to FIGS. 5A and 5B in the case where 4-level information is recorded.
FIG. 5A is a plan view of a groove where recorded marks are formed for a 4-level data sequence “3, 0, 3”, and FIG. 5B is a-diagram illustrating an RF (Radio Frequency) signal for forming the recorded marks.
Here, the recording power is set such that no recorded mark can be formed for the multi level 0 (Lv0). The recording power is set for the multi-level 0 so as to offset the inter-symbol interference. In this recording power correction, a reference signal level for the 4-level information recording power correction is set as the maximum signal level of the inter-symbol interference, that is, the signal level in the case where the multi level 0 is recorded between the multi levels 3.
Therefore, a threshold of the multi-level determination is determined by dividing a difference DR′ between the multi level Lv0 and the multi level Lv3 by 3. It is noted that the difference DR′ is smaller than a difference DR between the groove level GL and the multi level Lv3. If the interval between recorded marks is narrowed in the conventional multi-level information recording method so as to increase the recording density, the difference DR′ becomes smaller due to greater influence by the inter-symbol interference. In this case, since the level determination threshold has a shorter interval, the differences between multi-levels decrease and the margins become smaller. As a result, the multi-level determination becomes less accurate, that is, there is higher probability that the determination error occurs. As the number of multi-levels increases, the margins become smaller. Therefore, this problem becomes more serious.
In order to overcome the above-mentioned problem, a multi-level information recording method according to the present invention mainly intends to properly form a recorded mark in an information recording medium even if multi-level data are recorded in high density.
Furthermore, the multi-level information recording method intends to achieve other purposes. When recorded multi-level data are reproduced, a conventional multi-level recording method performs the following signal processing for removing inter-symbol interference.
FIG. 6 shows an example of recorded marks formed in a predetermined interval between adjacent recorded marks in a case where multi-level data “1”, “m” and “n” are recorded in the (i−1)-st cell through (i+1)-st cell in a track Tr, respectively. FIG. 7 shows waveforms before and after a waveform equalizing process for a reproduction signal of the recorded marks in FIG. 6. When the track Tr is played back, a reproduction signal s(i) of an i-th cell is supplied to a waveform equalizer as shown in FIG. 7. The waveform equalizer performs an operation based on the following equation (1) for the reproduction signal s(i),EQ(i|l, m, n)=C1{s(i)−s(i−1)}+s(i)+C2{s(i)−s(i+1)}  (1),where C1 and C2 are waveform equalizing coefficients. Then, the waveform equalizer outputs the waveform-equalized signal EQ(i|l, m, n). For instance, when a track including multi-level data “m” in the (i−1)-st cell through (i+1)-st cell is played back, the waveform equalizer outputs the waveform equalized signalEQ(i|m, m, m)=s(i)  (2),because the equations s(i)=s(i−1)=s(i+1) are satisfied for the equation (1) in this case.
In the equation (1), the waveform equalizing coefficients C1 and C2 are set so that the following formula
                              ∑                      l            ,            m            ,                          n              =              0                                7                ⁢                                  ⁢                              {                                          EQ                ⁡                                  (                                                            i                      |                      l                                        ,                    m                    ,                    n                                    )                                            -                              EQ                ⁡                                  (                                                            i                      |                      m                                        ,                    m                    ,                    m                                    )                                                      }                    2                                    (        3        )            can have the minimal value for all combinations of three 8-level data. If m-level information is recorded, the formula (3) is computed for all m3 combinations of m multi-level data. According to the above-mentioned waveform equalizing process, even if a reproduction signal has a waveform without sharpness due to the inter-symbol interference, it is possible to produce a sharp waveform signal by performing the waveform equalizing process.
However, there is a combination such that the equation EQ(i|l, m, n)=EQ(i|m, m, m) is not satisfied, because a recorded mark is not formed in a predetermined size due to thermal interference generated at recording time. In order to correct this problem, laser energy is adjusted for each combination of multi-level data.
A degree of the inter-symbol interference varies depending on the recording density, a shape of a laser beam and a type of an information recording media. In some cases, the above-mentioned conventional multi-level information recording method cannot properly correct the laser energy by simply using the difference between recorded multi-level data and the average of adjacent multi-level data.
In addition, since the conventional multi-level information recording method does not determine whether or not the recording power correction has succeeded, it is impossible to obtain certainty regarding appropriate reproduction of recorded multi-level data. Furthermore, while the correction is adjusted based on a recorded multi-level data sequence, it is determined whether or not recorded multi-level data are properly played back based on a reproduction signal. Thus, the recording power correction should be performed based on the reproduction signal rather than the recorded multi-level data sequence.
Accordingly, a multi-level information recording method according to the present invention further intends to achieve the following purposes. The first purpose is to provide a multi-level information recording method that can determine an optimal level of the laser energy with respect to combinations of multi-level data as small iteration times, that is, less computation as possible. The second purpose is to provide a multi-level information recording method that can prepare test patterns for determining laser energy corresponding to the combinations of multi-level data. The third purpose is to provide a multi-level information recording method that includes a laser energy setting method. The fourth purpose is to provide a multi-level information recording method that can easily control laser energy. The fifth purpose is to provide a multi-level information recording method that has broader tolerance with respect to multi-level data determination errors. The sixth purpose is to provide a multi-level information recording medium suitable to the multi-level information recording method according to the present invention. The seventh purpose is to provide a multi-level information recording-reproducing apparatus that can determine an optimal level of the laser energy with respect to combinations of multi-level data as smaller iteration times, that is, less computation as possible.